This invention relates to a serial interface system which comprises a master unit, a plurality of slave units, and a serial interface shared by the master and the slave units.
A serial interface system is described in U.S. Pat. No. 4,348,722 issued to Thomas G. Gunter et al and assigned to Motorola, Inc. Such a serial interface system may be used in portable terminal equipment on board a mobile vehicle, for example in a mobile communication network.
A conventional serial interface system may comprise a standard interface, such as one known in the trade by the code RC-232C. The RC-232C is an asynchronous interface. Another conventional serial interface system comprises a synchronous interface, such as used in a 72-chip central processing unit. The latter interface is operable at a high speed and has merits in hardware as well as in software when used in a one-to-one connection. The conventional serial interface must, however, comprise a chip select (CS) line for each unit when applied to a one-to-multiplicity connection. This makes it difficult to use the serial interface system to interconnect a master unit and five or more slave units. Moreover, sure serial interface system cannot flexibly accommodate a change in the connection or an increase in the number of slave units.